Heretofore, in the case of automotive vehicles, various reinforcing methods have been applied to steel plates which constitute the body thereof. For example, a plate comprising a metallic reinforcing material has been bonded to the inside of outer panels such as a roof, a fender, a hood, a trunk, a quarter panel and a door, which are relatively broad and have a flat form but have a thin thickness, by spot welding or with an adhesive, so that they have sufficient stiffness to withstand external forces exerted thereon. However, methods for reinforcing with such metallic reinforcing materials have various disadvantages, due to their heavy weight. For example, the weight of the outer panel, which is originally prepared so as to have a reduced thickness so as to decrease the weight of the vehicle body, is increased; also, the production costs are increased, and, furthermore, the steps for mounting such outer panels are complicated.
It is also known that in order to prevent vibration of the outer panels and also to reinforce them, polymeric materials such as asphalt rubber, an epoxy resin, an acryl resin, a phenol resin and an unsaturated polyester resin have been coated on or bonded to the back surface of the outer panels in a considerably large thickness and over a considerably large area. In this case, it is said that the stiffness is proportional to the cube of the thickness. Therefore, the stiffness of the outer panel can be increased by increasing the thickness of the coating. However, this method provides the same disadvantages as are encountered in the method for reinforcing with the metallic reinforcing material as described above, such as an increase in weight due to increasing the amount of the polymeric material, and an increase in production costs.
In view of these circumstances, U.S. Pat. Nos. 4,369,608, 4,378,385 and 4,444,818 have proposed reinforcing adhesive sheets which are lightweight and inexpensive that can greatly increase the stiffness of members to be reinforced.
Such reinforcing adhesive sheets comprise: (1) a thermosetting reinforcing resin sheet containing a reinforcing material in an unhardened or semi-hardened sheet; and (2) a bead-forming material bonded to the sheet (1), the material bring narrower than the sheet (1) and forming a bead-like projection prior to hardening the sheet (1), wherein portions of the sheet (1) extending beyond the bead-forming material (2) constitute adhesive surfaces of the reinforcing adhesive sheet to provide adherance to members to be reinforced.
The materials which can be used as the bead-forming materials are thermal recovering materials such as (a) a foamable resin sheet which expands upon heating to a temperature higher than the decomposition temperature of a foaming agent contained therein, (b) a flattened material which is prepared from a tubular material and which substantially recovers its original tubular form upon heating, and (c) a flattened material which is prepared from a non-tubular (e.g., rod- or film-shaped) material and which expands at least in a vertical direction upon heating, thereby substantially recovering its original non-tubular form.
The above flattened material prepared from a tubular or non-tubular material is produced by molding a thermoplastic polymer into a tubular form or a non-tubular form such as a rod-shaped form, preferably cross-linking the tubular or non-tubular material to provide a shape memory capability, and then flattened the material by techniques such as by passing the material through a hot press. The flattened material prepared from the tubular material is preferably designed so that a hot melt resin layer constitutes the inner surface, and this resin layer maintains the flattened state. The flattened material prepared from the non-tubular material is preferably such that the cross-linked film is stretched and the stretched film is fused to the sheet by means of, for example, a heat press.
A method of using the above-described reinforcing sheets can greatly improve the stiffness as compared with a method comprising bonding a sheet-like material having a uniform thickness and hardening the same. Such reinforcing adhesive sheets, however, have the following disadvantages.
Where the above-described foamable resin sheet is used as the bead-forming material, the shape of the foamed product obtained by heating the foamable resin sheet is not uniform, because the foaming pressure varies depending upon the heating temperature. This results in irregularities in the reinforcing effect. Further, since this method employs the technique that the foaming agent and resin are first kneaded and then molded into a sheet, it is difficult to use a foaming agent having a decomposition temperature of 120.degree. C. or less from the standpoint of the production process. It is therefore necessary for the temperature for forming the bead-like projection to be at least 130.degree. C., and preferably at least 140.degree. C. For this reason, sufficient reinforcing effect cannot be obtained under low temperature hardening conditions of 120.degree. C. or less.
Even where the flattened material prepared from the above-described flattened tubular or non-tubular material is used as the bead-forming material, the flattened material cannot sufficiently recover its original tubular or non-tubular form. In some cases, the recovered form varies depending upon the heating temperature. In particular, the recovering properties of the flattened material are not constant at low temperatures of 140.degree. C. or less and at high temperatures of 200.degree. C. or more. Thus, the reinforcing effect is irregular in such temperature ranges. Moreover, if the adhesion between the thermosetting reinforcing resin sheet and a member to be reinforced is increased, the original form cannot be completely recovered even though the appropriate heating temperature of from 140.degree. to 200.degree. C. is employed. This results in irregularities of the reinforcing effect.
Further, in order to recover the original tubular form of the flattened tubular material by heating the flattened material, a method has been employed, comprising treating the tubular material with electron beams and the like to increase the crosslinking density of the tubular material. If this treatment is not conducted, the flattened tubular material does not well recover its original tubular form.
However, this treatment is very difficult. For instance, if the treatment is insufficient, the flattened material does not recover its original form sufficiently, and if the treatment is conducted to excess, the tubular material becomes hard and it is difficult to flatten the tubular material. In addition, since the reinforcing adhesive sheet obtained using such a tubular material also becomes hard, it is difficult for the reinforcing adhesive sheet to sufficiently conform to a member to be reinforced having a curved surface and as a result, a sufficient reinforcing effect cannot be obtained.
Thus, it is recently difficult to mass-produce the tubular type material inexpensively and in the stable manner.